Wireless communication networks transmit communication signals in the downlink over radio frequency channels from fixed transceivers, known as base stations, to mobile user equipment (UE) within a geographic area, or cell. The UE transmit signals in the uplink to one or more base stations. In both cases, the received signal may be characterized as the transmitted signal, altered by channel effects, plus noise and interference. To recover the transmitted signal from a received signal, a receiver thus requires both an estimate of the channel, and an estimate of the noise/interference. The characterization of a channel is known as channel state information (CSI). One known way to estimate a channel is to periodically transmit known reference symbols, also known as pilot symbols. Since the reference symbols are known by the receiver, any deviation in the received symbols from the reference symbols (once estimated noise/interference is removed) is caused by channel effects. An accurate estimate of CSI allows a receiver to more accurately recover transmitted signals from received signals. In addition, by transmitting CSI from the receiver to a transmitter, the transmitter may select the transmission characteristics—such as coding, modulation, and the like—best suited for the current channel state. This is known as channel-dependent link adaptation.
Modern wireless communication networks are interference limited. The networks typically process transmissions directed to each UE in a cell independently. Transmissions to other UEs in the same cell are regarded as interference at a given UE—giving rise to the term inter-cell interference. One approach to mitigating inter-cell interference is Coordinated Multipoint (CoMP) transmission. CoMP systems employ numerous techniques to mitigate inter-cell interference, including MIMO channels, numerous distributed antennas, beamforming, and Joint Processing.
Joint Processing (JP) is a CoMP transmission technique currently being studied for Long Term Evolution (LTE) Advanced. In JP, transmissions to multiple UEs are considered jointly, and a global optimization algorithm is applied to minimize inter-cell interference. That is, JP algorithms attempt to direct transmission energy toward targeted UEs, while avoiding the generation of interference at other UEs. To operate effectively, JP systems require information about the transmission channels. There are two ways in which the channel information, or CSI, is fed back to system transmitters: Precoding Matrix Indicator (PMI) and quantized channel feedback.
PMI feedback, specified in LTE Release 8, is essentially a recommendation of a transmission format by each UE. A plurality of pre-defined precoding matrices are designed offline and known at both the base station and UE. The precoding matrices define various sets of downlink coding and transmission parameters. Each UE measures its channel, and searches through the precoding matrices, selecting one that optimizes some quantifiable metric. The selected precoding matrix is fed back or reported to the base station. The base station then considers all recommended precoding matrices, and selects the precoding and transmission parameters that implement a globally optimal solution over the cell. In the scenarios contemplated when Release-8 LTE was designed, PMI feedback works well, due to a high correlation between recommendations from UEs and the actual desirable transmission parameters. PMI feedback compression reduces uplink bandwidth by exploiting the fact that only part of the channel—the “strong directions,” i.e., the signal space—needs to be fed back to the transmitter.
In JP CoMP applications, it is unlikely that the desired transmission format (which achieves interference suppression) will coincide with a transmission format recommended by a UE. No recommending UE has any knowledge about other UEs that will be interfered by the transmission to the recommending UE. Additionally, the recommending UE has no knowledge of transmissions scheduled to other UEs that will interfere with its signals. Also, PMI feedback compression reduces bandwidth by reporting only the part of the channel of interest to transmissions directed to the recommending UE. While this increases uplink efficiency for non-cooperative transmission, it is disadvantageous for cooperative transmission, as it denies the network information about the channel that may be useful in the JP optimization.
In quantized channel feedback, UEs attempt to describe the actual channel. In contrast to PMI feedback, this entails feeding back information about not only the signal space but also the complementary space (the “weaker space,” also somewhat inaccurately referred to as the “null space”) of the channel. Feedback of the whole channel results in several advantages. With full CSI available at the network, coherent JP schemes can suppress interference. Additionally, the network can obtain individualized channel feedback by transmitting unique reference symbols to each UE. This enables flexible and future-proof implementations of a variety of JP transmission methods, since the methods are essentially transparent to the UE.
Even without JP CoMP transmission, CSI at the network can solve one of the most fundamental problems plaguing current wireless system—the inaccuracy in channel-dependent link adaptation due to the network not being able to predict the interference experienced by the UEs (a problem closely related to the well-known flash-light effect, as described by Afif Osserain, etc. in the paper “Interference Mitigation for MIMO Systems Employing User-specific, Linear Precoding,” PIMRC 2008). Once the network knows the CSI of bases near each UE, the network can accurately predict the SINR at each UE resulting in significantly more accurate link adaptation.
Even though the advantages of direct CSI over PMI feedback are clear, the major issue with direct CSI feedback is bandwidth. Full CSI feedback requires a high bitrate to transmit the CSI from each UE to the network. Time-frequency uplink channel resources must be used to carry the CSI feedback on the uplink channel, making these resources unavailable for transmitting user data on the uplink—the CSI feedback transmissions are thus pure overhead, directly reducing the efficiency of uplink data transmissions. Conveying direct CSI feedback to the network without consuming excessive uplink resources stands as a major challenge of modern communication system design.